The objectives of this research are to: a. Determine the effect of the nature of the substrate, its concentration and of the oxygen concentration on the half-reactions of monoamine oxidase (MAO) as studied with stopped-flow and spectral scanning procedures and, in particular, to determine how the anaerobic reactions, or both, differ in relative rates depending upon the nature of the substrate; b. Determine the site and nature of action of reversible inhibitors of MAO; e.g. harmaline; phenazine methosulfate; and tricyclic antidepressant drugs and, c. Ultimately compare the above parameters on MAO from beef liver with those of MAO types A and B from beef brain. The overall objective is to elucidate certain aspects of the fundamental nature of the MAO reaction and to obtain data which may prove useful in determining what role, if any, reversible inhibition of MAO may be involved in the action of clinically effective antidepressant drugs.